Interpretive Summary: Biomass fuels are being explored to replace or supplement nuclear and fossil fuels. Alfalfa is being investigated as a source of biomass for a biofuel production system where the stems are processed to produce biofuels and the leaves are sold as a source of animal feed. Previous research showed that delaying forage harvest until the alfalfa plants were starting to produce pods and decreasing the plant population to 40 percent of what is customarily used by growers and harvesting only twice per season would maximize leaf and stem yield for an alfalfa biofuel production system. However, as alfalfa matures the stem portion of the hay increases and the overall nutritive value of the forage declines. We investigated the effects of plant age and plant population number on leaf and stem nutritive value to make economic and management decisions about a biomass production system. Delaying harvest and reducing the stand density of the alfalfa plants resulted in minimal changes in leaf nutritive value and produced less digestible stems with higher protein concentration. We propose that decreasing stand density and harvesting only twice per season at a later plant age would maximize stem yield for biofuel production, increase the amount of nutritional leaf meal to be sold as an animal feed, and reduce costs in an alfalfa biofuel production system. This research will be useful to farmers who decide to manage alfalfa for biofuel uses.

Technical Abstract:
A system has been proposed using alfalfa (Medicago sativa L.) as a biofuel feedstock where the stems would be processed to produce energy and the leaves used as a livestock feed. A management regime reducing population density, delaying harvest, and cutting two times per season to maximize leaf and stem yield would be implemented for biomass production. Our objectives were to evaluate the effects and interactions of environment, population density, and harvest maturity on leaf and stem crude protein (CP), in vitro true digestibility (IVTD), neutral detergent fiber (NDF), neutral detergent fiber digestibility (NDFD), and CP yield of alfalfa germplasms differing in fall dormancy and leaf to stem ratio. Four alfalfa germplasms established at four population densities (450, 180, 50, and 16 plants m-2) were harvested at early bud and green pod maturity stages and evaluated in three environments for leaf and stem forage quality and CP yield. Germplasms varied for leaf CP and NDF concentration, but no differences were found for leaf CP yield, IVTD, or NDFD. Stem NDF, IVTD, and NDFD were higher in the dormant than the moderately dormant germplasms. Leaf and stem CP and NDF were unaffected by population density and were consistently higher in CP and IVTD and lower in NDF when harvested at early bud compared to green pod. No discernable response patterns to harvest maturities or plant densities occurred for leaf NDFD. Stem NDFD was greater at early bud, and both stem IVTD and NDFD decreased as population density decreased. When switching to a biomass management regime, alfalfa stems may be higher in NDF and CP yield, but lower in NDFD, IVTD, and CP concentration. Alfalfa leaves would be higher in NDF, lower in CP, and the same in NDFD and CP yield.